according to the pokemon entry in soulsilver version (generation four remake of pokemon silver (gen 2)) Lugia was said to be the guardian of the seas, hence, like palkia and dialga (as mentiond by brock in the animay) are tha guardians of time and space and arceus, like kyogre, is the ruler of time and space, kyogre of the seas, henceforth, ho-oh may actually be the guardian of the land as well as the sun, of course, considering oh-oh is associated with the sun, lugia should be associated with the moon, but i suppose in a way, lugia is associated with the moon because the moon (in real life) controls the tide and lugia has the ability the control the tied (considering it is the guardien and palkia & dialga can control time and space as guardians) and hence is associated with the moon.